1. Field of the Invention
This invention relates generally to an optical absorbing device and, more particularly, to a tapered amplitude, optical absorbing device, such as waveguide photodetector or electro-absorption modulator, in combination with a multi-mode interference coupler.
2. Discussion of the Related Art
Optical absorbing devices, such as photodetectors and electro-absorption modulators, are devices that operate by absorbing light. Photodetectors operate as optical to electrical converters. As a photodetector absorbs light, electron-hole pairs are generated which create an electrical photocurrent. A constant electric field is applied to the photodetector, and the photocurrent is a measure of the absorbed light intensity. Electro-absorption modulators operate as electrical to optical converters. A constant light intensity and a varying electric field are applied to the electro-absorption modulator. As the modulator absorbs the light, the electric field modulates the optical intensity of the light to convert the light intensity to a modulated intensity.
There are generally two device configurations for photodetectors and electro-absorption modulators. These configurations include a surface normal configuration and a waveguide configuration. When a waveguide configuration is employed, the light is typically input into a single mode waveguide having a dimension of approximately 2 .mu.m.times.2 .mu.m in cross-section. Light is absorbed by the device as the optical mode propagates along the waveguide. The absorption length typically varies between 25 .mu.m and 500 .mu.m.
The operation of photodetectors and electro-absorption modulators is limited by optical power saturation that may generate an excessively high electrical current density. The excessively high electrical current density can cause device failure. Because photodetectors are designed to absorb all of the light which falls on them, device failure caused by excessively high electrical current density is a significant problem. Also, saturation in a photodetector results in reduced responsitivity that affects the optical to electrical conversion efficiency. For electro-absorption modulators, saturation causes a reduced modulation efficiency and can in some cases result in reduced bandwidth and RF linearity. Although still a concern, device failure for electro-absorption modulators is less of a problem than saturation because they are designed to intensity modulate the light and only absorb a portion of the incident light. Therefore, optical saturation poses serious limitations for the usefulness of photodetectors and electro-absorption modulators.
Optical absorption is exponential for an absorbing device that incorporates a uniform waveguide. This exponential absorption is defined as A=Ke.sup.-.alpha.x, where .alpha. is the absorption constant and x is the distance along the device. The fundamental problem with existing waveguide photodetectors and electroabsorption modulators, as discussed above, is in this exponential absorption characteristic. The bulk of the light is absorbed at the beginning of the waveguide, with less and less absorption per unit length as the light propagates To overcome this problem, it is possible to design the device to have a very low absorption per unit length. The drawback to this approach, however, is that the device becomes very long, which limits bandwidth because of the RC limit.
What is needed is an optical absorption device, such as a photodetector or electro-absorption modulator, that does not suffer from the optical saturation problems discussed above. It is therefore an object of the present invention to provide an improved photodetector and electro-absorption modulator.